An electroluminescent panel is used as a flat panel display in many products. One common use is in a wrist watch. The power for driving the electroluminescent panel in a wrist watch is provided by a battery. A DC voltage must be converted into an AC voltage, which is then applied to the electroluminescent panel. However, it is necessary to maintain the frequency of this AC voltage at several hundred Hz and the peak-to-peak value of this AC voltage at several hundred volts because of the characteristics of the electroluminescent panel.
Existing power circuits that use the charging and discharging of a capacitor, in a circuit with an inductor, have been proposed as a power circuit to generate an AC voltage of several hundred volts from a DC voltage of tens of volts for use as described above. They are described in U.S. Pat. Nos. 4,208,869; 4,449,075; and 4,527,096.
Some existing power circuits for electroluminescence that use the charging and discharging of a capacitor, and power circuits for electroluminescence that use an inductor, generate rectangular waveforms. Therefore, they have the problem of a low luminous efficiency at the electroluminescent panel as well as the generation of a large amount of noise.
A power circuit for electroluminescence using a transformer has been developed in order to reduce the above problem. The electroluminescent panel, which is a capacitive load, makes up a resonance circuit in the power circuit using a transformer, and an AC voltage in a sinusoidal waveform is applied to the electroluminescent panel. Therefore, it has the advantages of reduced noise generation and high luminous efficiency.
However, the size of the transformer having a primary coil and a secondary coil is quite large, and the capacitance of the entire power circuit accordingly becomes large. Therefore, containing the power circuit within the housing of a wrist watch is difficult. Furthermore, the cost of the transformer is high, so increased cost of the entire power circuit presents another problem. Moreover, in order to emit a relatively large electroluminescence, it is necessary to increase the capacity of the transformer, and the transformer consequently becomes larger and heavier. Accordingly, power circuits for electroluminescence that use transformers are only used today in large size display panels and they cannot be used in small size applications, such as a wrist watch.